Accuracy of high precision resistors is important for many integrated circuit applications with, e.g., input/output (I/O) buffers. However, the accuracy of the actual integrated circuits to their mathematical models depends on foundry manufacturing processes used.
Often times, it is necessary to design high precision resistors before it is possible to validate the accuracy of the model versus a mature foundry manufacturing process. Further, it is a real challenge to change the process to meet the resistance accuracy requirements.
Any inaccuracy will require design changes and probably die size changes, which increase costs. In the end, it is often necessary to do layout redesigns and re-tapeout masks to meet the required resistance accuracy requirements. Die size change will impact overall integrated circuit ecosystem (abutment, density, routings, timing, etc.).
In the past, scalable resistors were often used to meet the required resistance accuracy requirements. Unfortunately, scalable resistors (P-cell) have certain limitations.
In new multi-gate or tri-gate semiconductor technologies, also known as FINFET technology, design rules restrict polysilicon gate lengths to 2 or 3 preset length dimensions so only width scaling is possible. This means a resistor footprint change is required if the resistance is off target. A footprint change means that the total die area will be impacted and changes to multiple masks would be required, e.g., metals, contacts, polysilicon, silicide block, and other masks.
Non-scalable or fixed-cell resistors also have been used. Non-scalable resistors are fine because they have a fixed footprint. Unfortunately, any discrepancy on the silicon die would require a whole template or fixed cell change. This again means a resistor footprint change is required if the resistance is off target. A footprint change again means that the total die area will be impacted and changes to multiple masks would be required, e.g., metals, contacts, polysilicon, silicide block, and other masks. The non-scalable or fixed-cell resistors can be connected in parallel or series to decrease or increase, respectively, resistance by a factor based on a number of the non-scalable or fixed-cell resistors.
Thus, a need still remains for a neutral area for any resistance changes, fewer mask changes to reduce cost, and the capability for finer resistance tuning. It is increasingly critical that answers be found to these problems. In view of the ever-increasing commercial competitive pressures, along with growing consumer expectations and the diminishing opportunities for meaningful product differentiation in the marketplace, it is critical that answers be found for these problems. Additionally, the need to reduce costs, improve efficiencies and performance, and meet competitive pressures adds an even greater urgency to the critical necessity for finding answers to these problems.
Solutions to these problems have been long sought but prior developments have not taught or suggested any solutions and, thus, solutions to these problems have long eluded those skilled in the art.